Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system with no known cause, but thought to be autoimmune in nature. It is estimated that more than 2 million people worldwide live with MS, and although substantial progress has been made in the development of better treatments, MS remains a chronic, incurable disease, and a significant cause of disability. The average life time risk of MS in U.S. women is about 5 per thousand. Because the peak age of onset is the late 20's to early 30's, the impact of MS extends over a critical period of life, and affects not only patients but also their children and extended families, so that prevention of even a small proportion of cases would have a large public health impact. The identification of modifiable risk factors that may reduce MS incidence is thus of tremendous importance. Converging evidence from multiple studies suggests that inadequate vitamin D nutrition, infection with the Epstein-Barr virus (EBV), and cigarette smoking are important risk factors for MS. Whereas smoking cessation can be comfortably promoted on a broad scale, existing evidence is still insufficient to recommend supplementation of healthy populations with the high doses of vitamin D that seem to be needed for MS prevention; and while an EBV vaccine has passed a phase-II trial, and seems to reduce the risk of clinical disease, it does not prevent EBV infection, and its effect on MS risk is unknown. Finland has among the highest rates of MS in the world and the Finnish Maternity Cohort (FMC), with over 1.5 million serum samples collected from over 800,000 pregnant women since 1983, provides a unique opportunity to examine these factors as predictors of MS, and how they may interact to determine MS risk, in a large group of women and their offspring. During the first trimester (at 10-14 weeks gestation) women provide a blood sample for screening of congenital infections. The serum leftover from these samples are stored by the FMC and are available for scientific research. The FMC includes ~98% of all pregnant women in Finland since 1983. Two related nested case-control studies are proposed here: 1) a study of pre-diagnostic serum vitamin D (25-hydroxyvitamin D) levels, EBV IgG antibody titers, and cotinine levels (a marker of cigarette smoking) and risk of MS among the mothers and 2) a study of in utero exposure to these factors during the first trimester of pregnancy and risk of MS in the offspring. The specific hypotheses to be tested among the mothers are that pre-diagnostic low vitamin D levels, elevated EBV IgG antibody titers, and elevated cotinine levels are associated with an increased risk of MS, and similarly among the offspring that in utero exposure to low vitamin D levels, elevated EBV antibody titers, and elevated cotinine will be associated with an increase risk of MS as an adult. By linking various hospital and drug registers with the FMC, we expect to identify and confirm 1,066 cases of MS among the mothers and 185 among the offspring (born between 1983 and 1991). Cases will be individually matched to 2 controls on age, time of sample collection, and place of residence (County); cases among the offspring will further be matched on date of birth and gender. Consistent with the matched case- control design, relative risks will be estimated using Mantel-Haenszel odds ratios; conditional logistic regression will be used for multivariate analyses. The study among the mother's proposed here will be the largest prospective study to date examining the individual and joint effects of vitamin D, EBV, and smoking on MS risk. No studies have been conducted examining the effect of in utero exposure to these factors and future risk of MS, and the FMC provides a unique opportunity to study these questions.